Erickson Observatory Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Erickson Observatory Site clear sky chart from 2003-08-01 to 2005-11-07.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	8% 66%	8665
	Night	9% 5% 60%	8503
Transparency	Day	18% 33% 40%	6883
	Night	5% 17% 33% 40%	6457
Seeing	Day	14% 40% 39% 5%	578
	Night	5% 17% 33% 35% 7%	2815

By Month

Percentage Cloud Cover
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month		Occurrence of each Forecast	Data Points
January	Day	18% 5% 46%	577
	Night	19% 5% 45%	775
February	Day	32% 6% 6% 6% 27%	561
	Night	30% 5% 5% 30%	684
March	Day	9% 6% 60%	662
	Night	8% 5% 5% 57%	699
April	Day	5% 67%	720
	Night	6% 5% 5% 7% 58%	600
May	Day	5% 67%	803
	Night	5% 5% 11% 57%	561
June	Day	91%	780
	Night	5% 83%	537
July	Day	5% 73%	789
	Night	6% 73%	564
August	Day	5% 5% 6% 67%	1114
	Night	5% 70%	929
September	Day	81%	993
	Night	79%	952
October	Day	11% 71%	649
	Night	8% 65%	714
November	Day	15% 56%	468
	Night	9% 5% 6% 56%	698
December	Day	23% 55%	549
	Night	21% 50%	790

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	6% 6% 32% 42% 12%	325
	Night	6% 6% 24% 42% 19%	431
February	Day	10% 6% 32% 33% 17%	215
	Night	12% 11% 32% 29% 15%	280
March	Day	10% 22% 34% 30%	454
	Night	6% 7% 21% 32% 31%	459
April	Day	8% 25% 37% 24%	565
	Night	21% 37% 33%	425
May	Day	8% 21% 30% 37%	658
	Night	8% 13% 28% 46%	423
June	Day	16% 37% 42%	737
	Night	17% 38% 39%	497
July	Day	13% 32% 46%	670
	Night	16% 31% 47%	473
August	Day	5% 6% 17% 28% 42%	888
	Night	5% 16% 36% 38%	735
September	Day	8% 22% 68%	913
	Night	7% 27% 63%	858
October	Day	14% 37% 42%	783
	Night	6% 16% 31% 45%	829
November	Day	8% 25% 37% 23%	332
	Night	7% 32% 28% 27%	546
December	Day	9% 35% 49%	343
	Night	9% 41% 43%	501

Seeing (when cloud cover <= 80%)
Color Meanings: Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month		Occurrence of each Forecast	Data Points
January	Day	28% 33% 38%	21
	Night	8% 20% 39% 26% 5%	202
February	Day	35% 54% 10%	37
	Night	15% 32% 35% 14%	154
March	Day	23% 30% 30% 14%	55
	Night	8% 22% 30% 33% 5%	220
April	Day	26% 50% 22%	50
	Night	9% 34% 32% 21%	218
May	Day	19% 48% 30%	56
	Night	5% 30% 39% 20%	181
June	Day	6% 45% 42% 5%	59
	Night	15% 38% 41%	180
July	Day	37% 57%	61
	Night	28% 57% 10%	190
August	Day	29% 60%	87
	Night	23% 53% 20%	352
September	Day	11% 42% 39% 6%	87
	Night	8% 34% 44% 11%	358
October	Day	10% 41% 35% 10%	65
	Night	11% 30% 44% 8%	349
November	Night	9% 28% 37% 21%	215
December	Night	11% 24% 37% 19% 6%	196

Page generated at 2008-08-29 13:02:59. Version ab11